The recent proliferation of satellite communication systems has increased the likelihood of interference between signals associated with neighboring satellites. Such interference can take place, for example, when a non-geostationary satellite comes within the field of view of a geostationary satellite. As is well known, geostationary satellites remain fixed in equatorial orbits over particular locations on the surface of the earth. Since geostationary satellites ordinarily exhibit some minor variation in latitude relative to the equatorial arc, there exists a narrow "geostationary band" centered about the equatorial arc corresponding to the set of orbital locations potentially occupied by geostationary satellites . Unlike geostationary satellites, the orbits of non-geostationary satellites continuously vary with respect to the earth's surface. Non-geostationary satellites typically traverse low and medium altitude orbits below the geostationary band.
Signal interference between geostationary and non-geostationary communication systems can result when non-geostationary satellites move into the field of view of ground stations oriented toward a particular satellite within the geostationary band. The potential for such interference arises whenever a non-geostationary satellite becomes located proximate the feeder link path between a geostationary satellite and one of its ground stations, hereinafter referred to as GSY ground stations. Such interference can occur because non-geostationary satellite systems are generally allocated, on a secondary basis, the same feeder link frequency bands primarily earmarked to geostationary systems. Consequently, it is incumbent upon the operators of non-geostationary systems. Consequently, it is incumbent upon the operators of non-geostationary systems to avoid disrupting communication within geostationary systems. Although it is conceivable that the feeder link band could be shared by geostationary and non-geostationary systems, the frequency separation required between the channels allocated to each system in order to ensure acceptable interference levels would make this approach unfeasible under most circumstances.
Since geostationary satellites are distributed throughout the geostationary band above the surface of the equator, the points on the surface of the earth in approximate alignment with the geostationary band and a non-geostationary satellite form a range of "in-line" latitudes across the earth's surface. The position of this terrestrial in-line latitude range will vary with changes in the latitude of the non-geostationary satellite. Yet non-geostationary satellites may interfere with geostationary systems even when not so aligned between a geostationary satellite and a GSY ground station, since the antenna of the GSY ground station projects a radiation pattern across a finite discrimination angle relative to its beam axis. Accordingly, it has generally been necessary for non-geosynchronous satellites to cease signal transmission when in orbit above GSY ground stations in the vicinity of this in-line latitude range. This restriction on transmission range has hindered the performance of non-geostationary satellite systems coordinated in frequency with geostationary systems.
One way of minimizing interference between satellite systems would be simply to operate one system over frequency bands not already allocated to the other systems. Unfortunately, the limited frequency spectrum available for satellite communication systems renders this solution untenable. Moveover, well-established technology is available for implementing communications equipment designed to process signals over the frequency bands primarily allocated to geosynchronous systems.
While most satellite systems have in the past used geostationary or geosynchronous satellites, the future development of systems using low and medium earth orbits is likely to increase the problem of interference, not only between geostationary and non-geostationary systems, but also between two or more non-geostationary systems.
U.S. Pat. No. 5,227,802 discloses a method of controlling the amount of overlap between cells projected by different satellites, by turning off cells from one satellite when they overlap by more than a predetermined amount with cells from another satellite.